Multi-stage flash evaporator with heat recovery



1964 T. M. CHALMERS ETAL 3,146,177

MULTI-STAGE FLASH EVAPORATOR WITH HEAT RECOVERY Filed Nov. 3, 1960 2 Sheets-Sheet 2 1 a 36L k Es g8 I: g 2% il \I b 4 lnvenlor Attorney United States Patent 3,146,177 MULTI-STAGE FLASH EVAPORATOR WITH HEAT RECOVERY Thomas Martin Chalmers, Shiremore, and Adolf Frankel and Hugh Robert Morton Craig, both of Newcastle-on- Tyne, England, assignors to Richardsons, Westgarth 8: Co. Limited, Wallsend, Northumherland, England Filed Nov. 3, 1960, Ser. No. 67,133 Claims priority, application, Great Britain, Nov. 6, 1959,

37,77 9/ 59 2 Claims. (Cl. 202-160) This invention relates to multi-stage flash-type evaporators, and particularly to evaporators for distilling salt water or brackish water.

Evaporators of this type are provided with a number of flash chambers each of which is associated with a heater. Each flash chamber and its associated heater may be arranged separately from one another and be joined by a connecting pipe; alternatively, the flash chambers may be arranged in a common vessel and the heaters in another common vessel separate from but joined to the flash chamber vessel. A more recent development, however, it to arrange the heaters and flash chambers in a single vessel which is divided into compartments by partition walls, and the present invention is of particular advantage when applied to an evaporator of this design.

Each heater of such an evaporator is provided with one or more tubes for conveying therethrough a liquid to be evaporated. The liquid. during its passage through the tube or tubes, is heated by vapour which is flashed oif in each of the successive flash chambers and which condenses on the tubes. The amount of heating surface required in an important factor for determining the overall length of the tubing, so that the dimensions of the part of the vessel housing the heaters may be said to be substantially a function of the length of tubing it is desired to arrange therein. The best size for the part of the vessel housing the flash chambers, however, is independent of the overall length of the tubing and thus of the heater part of the vessel. Accordingly, it is an aim of the present invention to provide an evaporator which allows a designer to take such considerations into account to a far greater extent than has been possible in earlier designs of evaporator.

According to the invention, a multi-stage flash-type evaporator comprises a group of two or more flash chambers, each of which is connected to an associated heater provided with one or more tubes for the passage therethrough and the heating therein of liquid to be evaporated in the flash chambers, in which the flash chambers in the group are connected together in series while at least two of the heaters are connected together in parallel so that, in operation, the liquid to be evaporated is divided into a number of parallel streams prior to its entry into the heaters.

The flash chambers may be smaller than the heaters so that the respective sizes of the flash chambers and the heaters approach more closely the dimensions which give the best results in practice. In the best construction, the flash chambers and associated heaters of each group form a compact unit or section of regular shape-for example, a rectangular unit.

Some examples of evaporators in accordance with the invention will now be described in more detail with reference to the accompanying drawings, in which:

FIGURE 1 is a diagrammatic view of a flash-type evaporator which is outside the scope of the invention;

FIGURES 2 and 3 are diagrammatic views of two different forms of evaporator according to the invention; and

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FIGURE 4 is a diagrammatic view of a modification to the evaporators shown in FIGURES 2 and 3.

Referring now to the diagram shown in FIGURE 1, the evaporator comprises a number of flash chambers 10 each of which is associated with a heater 12. All the flash chambers are connected in series through openings 14, as are the tube nests 16 arranged in each of the heaters. The arrows D indicate the directions of flow through the evaporator of a single stream of liquid to be evaporated. The liquid, before being transferred from the last heater 18 to the first flash chamber 20, is conveyed through a heat-input section 22 in which its temperature is raised to produce a temperature difference between the liquid in the last heater 18 and the first flash chamber 20 which is suflicient for flashing to take place in the said flash chamber. The arrows C indicate the flow path through the openings 24 of the vapour flashed off in each of the flash chambers 10. The size of each flash chamber 10 is substantially equal to the size of each heater 12, and the tube surfaces in the heaters are substantially equal and provide a predetermined amount of heating surface.

In contrast to the earlier design of evaporator shown in FIGURE 1, the evaporator illustrated in FIGURE 2, although having the same number of flash chambers and heaters, has the flash chambers 10 and the heaters 12 arranged in six sub-groups each having two flash chambers 10a, 10b and two heaters 12a, 12b respectively so that some of the liquid flows through the tube nest 16a arranged in the group of heaters 12a while the remaining liquid flows parallel to that stream through the tube nest 16b arranged in the group of heaters 12b. The two streams are recombined by short pipes 29 after having passed through the heaters of each sub-group, but are divided again before they enter the heaters of the following sub-group. The two streams leave the final heaters through takes 17 and 19 and are recombined finally into the tube 21 for their flow through the heat-input section 22 and hence through the flash chambers 10a and 10b located closest to the heat-input section. The flash chambers Ida and 10b of the six sub-groups are connected in series through the openings 14 and the pipe connections 26 to form two main groups, and the liquid not flashed off in the first flash chambers is transferred to the following flash chambers until it finally reaches the flash chambers located on the extreme right-hand side of the figure and leaves through the outlet 27. Condensate is removed from the heaters through outlet pipes 23 and 25, while liquid to be evaporated enters through the inlet pipe 29.

The evaporator shown in FIGURE 3 is similar to the one illustrated in FIGURE 2 except that, in addition to the six groups of series-connected flash chambers 10a and 10b, their associated heaters 12a and 12b respectively are also connected in series to form two main groups of heaters. As a result, the-two parallel streams of liquid flow through each of their respective heaters 12a and 12b and are recombined only once for the passage through the heat-input section 22 and the flash chambers.

In contrast to the evaporator shown in FIGURE 1 in which successive flash chambers 10 are associated with heaters 12 following one another in series, each flash chamber ltla of the evaporators shown in FIGURES 2 and 3 is associated with a heater 12a, while each flash chamber 10b is associated with a heater 12b.

While the arrangement of flash chambers and heaters will generally be such that the ratio of the quantity of liquid in each of the parallel streams to the total quantity is substantially the reciprocal of the number of streams, there may be occasions when it is more advantageous for the quantities of liquid in the various streams to differ from one another. In this instance, the tube surfaces in the heaters may also vary either from heater to heater or between the heaters in one group, or set, and the heaters in another group 'or set.

The invention is not restricted to the main part of an evaporator. For example, most evaporators include a heat-rejection section for reducing the temperature of the part of the unevaporated liquid that is recirculated through the heaters. In its simplest form, the heat rejection is effected by piping the unevaporated liquid through a vessel so that it gives up heat to cold water, for example, which is pumped continuously through the vessel and which, after absorbing the heat, is discharged to waste. However, like the main part of the evaporator shown in FIGURE 2 or FIGURE 3, the heat-rejection section may also consist of a number of flash chambers with associated heaters. In this instance, the cooling water conveyed through the heat-rejection section may also be divided into parallel streams. Such an arrangement is shown in FIGURE 4 where the heat-rejection section 28 consists of four flash chambers 30 divided into two groups of two flash chambers each, the cooling water being divided into two parallel streams 32 and 34 which pass through heaters 36 and 38 respectively.

One of the advantages to be derived from extending the multi-flow principle according to the invention to the heat-rejection section 28 will be evident from the following consideration: The heat-rejection section of a conventional evaporator is invariably designed for a specific cooling Water temperature, for instance, the temperature of sea water. If, however, as a result of a reduction of this temperature the bottom temperature of the unevaporated liquid drops, the specific volume of vapour flashed off in the last flash chamber increases. As a further result, the vapour volume to be handled by the heater associated with this flash chamber increases considerably and there is a risk of an increased carry-over of liquid particles in the vapour generated in the flash chamber. In order to prevent this from happening, it is usual to arrange for some of the cooling Water to bypass at least the heater associated with the last flash chamber. However, if more than two flash chambers with their associated heaters are used in the heat-rejection section, the control problem rapidly gets out of hand if full advantage is to be taken of cooler sea water conditions and if, simultaneously, the aforementioned increased carry-over of liquid is to be avoided.

In contrast to this situation, the work to be done in each of the flash chambers and associated heaters forming the heat-rejection section remains substantially constant if, as shown in FIGURE 4, the cooling liquid is arranged to flow in a number of parallel streams 32 and 34 through this section. In this case, the last flash chamber does not operate under a substantially higher load than the other flash chambers. Effective control may be achieved simply by throttling the whole of the cooling water supply with one valve 40 instead of using a number of interstage valves or by-passes as in conventional evaporators. If desired, the valve 40 can be operated automatically by means 42 sensitive to the temperature of the cooling liquid or by means 44 sensitive to the temperature of the liquid in one of the flash chambers.

One of the advantages of an evaporator according to the invention is that a smaller number of tubes of greater length may be used which permits a saving in manufacturing costs. In addition, it will be found that, in many instances, less tubes of greater lengths may be accommodated more easily to the overall design of the evaporator. Further, the water boxes required in an evaporator according to the invention may be of a more simplified design.

We claim:

1. A multi-stage flash type evaporator comprising a first group of flash chambers and associated heat exchangers acting as heaters, and liquid conveying conduits for liquid to be heated, said heaters being arranged in at least two series, there being a plurality of heaters in each series, the heaters in each series being serially connected for the flow of condensate from the heater at one end of said series through the intermediate heaters, to the heater at the other end of said series, said one end corresponding to that end where liquid heating is terminated and flash vaporization begins and said other end corresponding to that end where heating begins and flash vaporization terminates, a condensate outlet from each heater discharging into the next upstream heater with regard to feed flow and discharging at said other end of each series, said flash chambers being equal in number to said heaters, alternate ones of said flash chambers being connected for the flow of vapor to separate and corresponding heaters of one series of said heaters in degrading thermal relation and the remaining alternate flash chambers being similarly connected to the other series of heaters, said flash chambers being all serially arranged and connected for the flow of liquid to be evaporated and unevaporated liquid from the flash chamber at said one end of said series through the intermediate flash chambers to the flash chamber at said other end of said series, a discharge outlet for unevaporated liquid from the flash chamber at said other end of said series, said liquid conveying conduits, one for each series, for liquid to be heated each extending through a separate series of heaters from a common inlet adjacent said other end thereof at which condensate is discharged to said one end thereof, a common conduit connecting said liquid conveying conduits adjacent said one end of said series of heaters, said common conduit discharging into the flash chamber at said one end of the series, a heater positioned to supply heat to the liquid flowing in said common conduit and a second group of flash chambers and conduits for liquid to be treated and associated heat exchangers acting as condensers similar to said first group and connected to said first group such that liquid conduits of said second group are connected to a source of cooling liquid and the flash chambers of the second group receive the unevaporated liquid from the first group whereby said second group operates as a heat removal section.

2. A multi-stage flash type evaporator as defined in claim 1 comprising a control valve in the common inlet for liquid to said second group of heaters, said valve be 1 ing connected to and under the control of means sensitive to temperature in one of said second group of flash chambers.

References Cited in the file of this patent UNITED STATES PATENTS 1,948,052 Smith Feb. 20, 1934 2,073,825 Beck et a1. Mar. 16, 1937 FOREIGN PATENTS 1,090,781 France Apr. 4, 1955 748,572 Great Britain May 2, 1956 

1. A MULTI-STAGE FLASH TYPE EVAPORATOR COMRISING A FIRST GROUP OF FLASH CHAMBERS AND ASSOCIATED HEAT EXCHANGERS ACTING AS HEATERS, AND LIQUID CONVEYING CONDUITS FOR LIQUID TO BE HEATED, SAID HEATERS BEING ARRANGED IN AT LEAST TWO SERIES, THERE BEING A PLURALITY OF HEATERS IN EACH SERIES, THE HEATERS IN EACH SERIES BEING SERIALLY CONNECTED FOR THE FLOW OF CONDENSATE FROM THE HEATER AT ONE END OF SAID SERIES THROUGH THE INTERMEDIATE HEATERS, TO THE HEATER AT THE OTHER END OF SAID SERIES, SAID ONE END CORRESPONDING TO THAT END WHERE LIQUID HEATNG IS TERMINATED AND FLASH VAPORIZATION BEGINS AND SAID OTHER END CORRESPONDING TO THAT END WHERE HEATING BEGINS AND FLASH VAPORIZATION TERMINATES, A CONDENSATE OUTLET FROM EACH HEATER DISCHARGING INTO THE NEXT UPSTREAM HEATER WITH REGARD TO FEED FLOW AND DISCHARGING AT SAID OTHER END OF EACH SERIES SAID FLASH CHAMBERS BEING EQUAL IN NUMBER TO SAID HEATERS, ALTERNATE ONES OF SAID FLASH CHAMBERS BEING CONNECTED FOR THE FLOW OF VAPOR TO SEPARATE AND CORRESPONDING HEATERS OF ONE SERIES OF SAID HEATERS IN DEGRADING THERMAL RELATION AND THE REMAINING ALTERNATE FLASH CHAMBERS BEING SIMILARLY CONNECTED TO THE OTHER SERIES OF HEATERS, SAID FLASH CHAMBERS BEING ALL SERIALLY ARRANGED AND CONNECTED FOR THE FLOW OF LIQUID TO BE EVAPORATED AND UNEVAPORATED LIQUID FROM THE FLASH CHAMBER AT SAID ONE END OF SAID SERIES THROUGH THE INTERMEDIATE FLASH CHAMBERS TO THE FLASH CHAMBER AT SAID OTHER END OF SAID SERIES, A DISCHARGE OUTLET FOR UNEVAPORATED LIQUID FROM THE FLASH CHAMBER AT SAID OTHER END OF SAID SERIES, SAID LIQUID CONVEYING CONDUITS, ONE FOR EACH SERIES, FOR LIQUID TO BE HEATED EACH EXTENDING THROUGH A SEPARATE SERIES OF HEATERS FROM A COMMON INLET ADJACENT SAID OTHER END THEREOF AT WHICH CONDENSATE IS DISCHARGED TO SAID ONE END THEREOF, A COMMON CONDUIT CONNECTING SAID LIQUID CONVEYING CONDUITS ADJACENT SAID ONE END OF SAID SERIES OF HEATERS, SAID COMMON CONDUIT DISCHARGING INTO THE FLASH CHAMBER AT SAID ONE END OF THE SERIES, A HEATER POSITIONED TO SUPPLY HEAT TO THE LIQUID FLOWING IN SAID COMMON CONDUIT AND A SECOND GROUP OF FLASH CHAMBERS AND CONDUITS FOR LIQUID TO BE TREATED AND ASSOCIATED HEAT EXCHANGERS ACTING AS CONDENSERS SIMILAR TO SAID FIRST GROUP AND CONNECTED TO SAID FIRST GROUP SUCH THAT LIQUID CONDUITS OF SAID SECOND GROUP CONNECTED TO A SOURCE OF COOLING LIQUID AND THE FLASH CHAMBERS OF THE SECOND GROUP RECEIVE THE UNEVAPORATED LIQUID FROM THE FIRST GROUP WHEREBY SAID SECOND GROUP OPERATES AS A HEAT REMOVAL SECTION. 